1. Technical Field
This disclosure relates to light emitting diodes (LEDs), dimmer controls, flyback controllers, and power factor correction.
2. Description of Related Art
Cold cathode fluorescent lamps have long-since been used in offices and have become popular in the home. Compared to incandescent lamps, their lumens per watt may be very high, saving energy. However, they may require a high voltage AC inverter and may contain toxic mercury.
Light-emitting diodes (LEDs) are also now capable of providing high light output per watt, comparable to cold cathode fluorescent lamps. Unlike cold cathode fluorescent lamps, moreover, they may not require high voltage and do not usually contain mercury.
Driving LEDs from the 110 volt alternating line current that is typically available, however, may be challenging. Unlike incandescent lamps, for example, the intensity of an LED may be proportional to the current which is delivered through it, not the amount of voltage that is applied across it. Thus, circuitry may be needed to convert the line voltage to a constant current. It may also be desirable to configure this circuitry so that it may drive the LED from the output of a conventional dimmer control, such as one that uses a triac.
One approach has been to convert the output of the dimmer control to a constant current using a flyback converter. Some dimmer controls, however, leak current during periods when their triacs are not firing, i.e., during off periods of their chopped AC voltage output. This leakage may cause a voltage build-up in the flyback controller, which may cause noise, flickering, and/or other undesirable results.
Dedicated hold-off circuits with low current limits have been added in an attempt to address this problem. These circuits may prevent the supply voltage from increasing to a point that might otherwise cause the flyback converter to be energized. However, these dedicated hold-off circuits may require an additional high voltage active device, such as a bipolar transistor. This may add costs, complexity, and size.